The present invention relates to a semiconductor device having the function of controlling touch detection on a touch panel, a method for canceling the periodic noise influence on a periodic signal capturing action, and a technique useful in application to a semiconductor control device operable to control e.g. a touch panel integrated with a liquid crystal display panel.
The applicant has proposed a technique that may be used for preventing a decrease in the accuracy of touch detection owing to extraneous noise, such as AC charger noise of a portable information terminal device having a touch panel incorporated therein in the related art (Japanese Patent Application No. 2012-216745), which has not been laid open yet. Specifically, the output voltage of an AC charger can fluctuate together with the local ground voltage of the AC charger at a predetermined frequency. Regarding the global ground outside the touch panel with respect to the local ground, the above phenomenon may cause the global ground to also fluctuate at the same frequency. Therefore, with a finger in conduction with the global ground or the like brought close to the touch panel, a surface voltage which acts on the touch panel through a stray capacitance produced by the finger or the like is periodically changed in synchronization with this frequency. In case that the cycle of the change coincides with the touch detection cycle, or is an integer multiple thereof, a particular voltage is periodically applied to the detection circuit, and as a result of the accumulation of such voltage, considerable noise is produced, which ends up worsening the accuracy of touch detection. The extraneous noise problem (AC charger problem) like this is based on the assumption that the accuracy of touch detection is worsened by noises caused by periodically scanning a display panel and by gradation drive signal changes unless the timing of touch detection is changed relative to the periodic display scanning and the gradation drive signal changes. This point must be taken into consideration. Therefore, making an attempt to solve the AC charger problem by changing the unit of a touch detection cycle with respect to display scan and gradation drive cycles, the touch panel is put in danger of being influenced by noises caused by scan and gradation drive signal changes. Hence, the related art described above makes it possible to select a detection period in each cycle of the integrating action without switching the frequency of performing the detecting action. The technique according to the related art cannot solve the problem: the AC charger problem cannot be solved by simple application of a technique arranged so that a touch panel is driven by using drive pulses that are of different frequencies in order to reduce noise in a detection cycle of an electrostatic capacitance type touch panel and parts of results of detection which suffer when just a small influence of noise is present.
An example of the above technique in connection with an electrostatic capacitance type touch panel is disclosed in the International Patent Publication No. 2009-535742.